The present invention relates to a process for forming a novel anticorrosive coating on Mg alloy, to an MG alloy member and household electrical appliances, audio systems, etc. using materials with such an anticorrosive coating, and more particularly to a Mg alloy member having a good corrosion resistance given by an environmentally harmless chemical conversion treatment, its use, a solution for chemical conversion treatment and a process for anticorrosive coating.
Mg alloy materials have the lightest weight among the practical metallic materials and also have a large specific strength and a good castability, and thus their wider application to cases, structural bodies, various parts, etc. of household appliances, audio systems, aircrafts, automobiles, etc. has been desired. Particularly, Al-containing AZ91D (Al: 8.3-9.7wt. %) and AM60B (Al: 5.5-6.5 wt. %) have a good fluidity in die casting and thixo molding and thus are most desirable alloys.
However, Mg has the basest normal electrode potential among the practical metallic materials, resulting in a high corrosion susceptibility when the metal is brought into contact with other metals and a considerably poor anticorrosiveness in an aqueous acidic, neutral or chloride solution. Thus, for its application to corrosion-excluding positions, e.g. good appearance-maintaining positions etc., it is necessary to provide an anticorrosive treatment. Coating is the most popular anticorrosion means, but it is hard to apply coating to Mg alloy materials per se because of the disadvantage that the resulting coating film, even if obtained has a poor adhesiveness. Sometimes, corrosion may occur under the coating film, and thus it is the ordinary practice to conduct a substrate surface treatment in advance of the coating process.
The substrate surface treatment technology includes, for example, substrate surface treatments of forming a metal oxide film or a sparingly soluble salt film by chemical conversion treatment or anodizing using such heavy metal oxo acid salts as chromates, permanganates etc., or phosphates so as to improve the corrosion resistance and the adhesiveness of coating films.
It is also the ordinary coating practice to use oil paints and synthetic resin paints which contain lead compounds, zinc powder and its compounds, chromates, etc. as an anticorrosive pigment.
Processes for forming an anticorrosive film on a Mg alloy are disclosed in JP-A-9-176894 and JP-A-9-228062.
Surface treatments using specific chemical compounds such as chromates, permanganates, etc. however have problems relating to environmental friendliness, such as effluent water pollution problems and skin allergy problems to operators. The use of such surface treatments is increasingly subject to strict regulations. Phosphates are also more or less harmful to the environment, and the corrosion resistance of resulting phosphate films is not satisfactory. Substitute processes for such substrate surface treatments are now under development but still have problems with respect to corrosion resistance, etc.
Lead compounds or chromates contained as anticorrosive pigments in coating technology also have problems relating to environmental friendliness. Furthermore, there are occasionally problems relating to corrosions probably due to diffusion of oxygen or water generated by corrosion under the coating film or by coating film defects.
The invention disclosed in said JP-A-9-176894 relates to an electrolytic treatment. Anodizing requires a power source of high voltage. An entirely uniform film is also hard to obtain. In the invention disclosed in said JP-A-9-228062, treatments using an organometal are highly reactive and thus an entirely uniform film is likewise hard to obtain.